Scientist's Work Makes Men Squirm

April 07, 1999|By Mary Roach, Discover magazine.

`Penis, penis, penis, penis, penis." Diane Kelly is making the point that she is used to saying the word. She says it in front of large gatherings of academics. She has said it to a Tallahassee police officer who caught her by the side of the road, removing one from a dead armadillo. She says it to people at barbecues and dinner parties who ask her what she does for a living.

Kelly, a 30-year-old postdoctoral fellow at Cornell University, is the first, and to her knowledge only, scientist in the world to study the biomechanics of the mammalian penis.

It is easy to speculate on the reasons no scientist has studied the penis in the past: a) the majority of scientists are men, and b) the research involves things like "inflation and bending experiments," wherein a rat penis is attached, by way of electric clamps, needles and superglue, to a strain gauge and a force transducer. (The organ is not, mercifully, attached to the rat.) Kelly allows that "men do squirm a lot" at her talks.

Her field, biomechanics, the application of principles of physics and engineering to living things, is a young science, only 35 years old or so.

With biomechanics' elders heading into retirement, a second generation has taken the baton and carried it into less obvious places: the biomechanics of hearing, of smelling, of circulation. Kelly was one of the intrepid few to claim reproduction.

She chose male reproductive organs over female not because of any prurient interest, she says, but because they're so much simpler: fewer parts, fewer functions. The same practical bent determines the species she studies.

Dead armadillos, her original organ donors, were easy to come by in the part of Florida where she did her graduate work. When armadillos are startled, they invariably jump into the air, putting them at grille height and pretty much sealing their fate in the face of oncoming traffic.

More important, the male Florida armadillo has the advantage of being uncommonly well endowed. Its penis can be as long as one-third its body length (a proportion that stretches to two-thirds in some South American armadillos), which may have something to do with the need to insinuate it under the female's shell.

Biomechanically speaking, the male reproductive organ has to do two things. It has to get rigid, and it has to deliver sperm. Kelly made her name in the rigidity arena, solving, as she puts it, "the mechanical problem of how to make something that's floppy stiff."

The answer she found contradicted conventional biomechanical wisdom. A penis is, in its materialist essence, a fleshy cylinder, and as such it's far from unique in nature. Mechanically, it's akin to an earthworm or a squid tentacle.

Instead of relying on bones for support, these "hydrostatic" structures use pressurized liquid. Hydrostats are elegant engineering solutions to a host of physical challenges, but they necessitate a couple of safety mechanisms.

First, a structure that relies on internal pressure for stiffness must have a means of restraining that pressure -- a tough skin -- to make sure its innards stay innards.

Second, if it's going to bend, it must bend without crimping.

In case after case, the solution nature has provided for these cylinders is to protect them on the outside with fibers of collagen woven in crossed helices, like intertwined spirals of DNA. This arrangement lets the cylinder resist the internal pressure yet bend smoothly and spring back naturally to its original position, a desirable state of affairs if you're, say, a worm on the move.

Researchers had assumed that penises must have the same arrangement of fibers. Apparently, no one had argued that a penis is not a worm. A worm, though, if it's going to get anywhere, needs to be readily bendable. A penis, for reasons that do not need explaining, must be rigid. It has one place it needs to go, and wriggling is not how it gets there.

Kelly discovered, while examining armadillo penises for her dissertation, that the fibers in the mammalian penis are not in fact helically wound. Instead they're laid out in layers: one longitudinal, in which the fibers run parallel to the organ's long axis, and one circumferential, in which the fibers run perpendicular. Kelly called the arrangement an axial orthogonal array.

Having collagen fibers arranged this way discourages bending and allows the erect penis to hold its shape. It's not squishable.